The production of etched semiconductor wafers for use in microcircuit chips typically employs large process module systems. Semiconductor process modules form part of a large grouping of components that are usually located in a clean room environment. A clean room is an isolated environment in which the introduction of dust and other foreign matter is strictly controlled. Only by maintaining such control can a high quality semiconductor product be produced. The high level of environmental control makes clean room space extremely expensive. Since the majority of semiconductor processing equipment must be located within the floor space of the clean room, the size of that equipment becomes a significant cost consideration.
A conventional semiconductor wafer processing facility, according to the prior art, is detailed in FIG. 1. The facility in this example is a cluster tool 20. A variety of arrangements and organizations of facilities are in use. In particular, the cluster tool includes a central transport or handling module or platform 22 that is surrounded by modules that perform operations on the wafers. In this example, unprocessed wafers 24 are stored in a vacuum cassette elevator 26 that moves a stack of wafers upwardly to provide new wafers as needed. A robot arm 28 having articulating joints 30 and 32 connecting arm segments 33 and 35 is used to handle wafers 24 within the facility 20. An end effector 34 engages each wafer 24 and withdraws it (double arrow 36) from the vacuum cassette elevator 26 as needed. The arm 28 moves about its pivoting base 38 to access each of the facility modules according to a preprogrammed sequence or "recipe."
Wafers 24 are processed to include etched or deposited surfaces by process modules that perform specific tasks. A typical cluster tool can include two to four process modules. Wafers are often shuttled between one process module and another to undergo multiple process steps. Prior to entry of a wafer, into a process module it is common practice to align and orient the wafer into a standard position. Most wafers are circular in perimeter outline, but each may include a notch, flat or other "interruption" at an arbitrary location along its perimeter. This notch or flat serves to provide a standard rotational reference point for further processing.
When wafers 24 are stacked in the cassette 26, they are not generally oriented so that all notches or flats are similarly positioned. In addition, wafers are not always accurately centered on the end effector 34 following retrieval from the cassette 26. Hence, without performing prior alignment, each wafer may enter a particular process module with its own unique rotational orientation and centering. Proper processing generally necessitates that wafers be centered in the various process modules. Thus they must be centered relative to the end effectors that place wafers into the modules. Rotational orientation should also remain constant, particularly during the photolithography step of the process so that patterns are stacked over one another in the proper relationship. It is also desirable to maintain a known rotational orientation when wafers are periodically inspected for non-uniformities. An inspection usually entails a survey of the processed surface with respect to the notch or flat. If some portion of the processed surface includes a non-uniformity or defect, then this may suggest that a particular process module should be serviced. Accordingly, the position of the notch or flat relative to the mounting location 41 and 43 of the respective process module 40 and 42 must be known to derive the source of the non-uniformity. Hence, prior to insertion of wafers 24 into process modules 40 and 42, the wafers are fed (arrow 44) to an alignment module 46 that places each wafer in a properly centered and rotated position with respect to the robot arm.
During the alignment step, the end effector 34 lays each wafer atop a spinner or capstan 48 having a series of vacuum ports 50, or other friction-generating devices that retain the wafer on the spinner 48. A lifter having four pins 52 can be employed to carefully lower the wafer onto the spinner once the end effector is located over it. FIG. 2 illustrates the positioning of a typical wafer 24, in an unaligned state, on the spinner 48. The wafer's center 54 is offset from the desired center 56. Likewise, the notch 58 is offset from the desired rotational positioning of the notch (shown in phantom). This rotational offset is represented by an angle .theta.. The spinner 48 rotates the wafer 24 as an edge detector 60 scans the perimeter 62 of the wafer. Changes in position of the detected perimeter edge during rotation are used to derive the eccentricity of the wafer center 54 relative to the spinner center 56. An edge detect circuit 66 calculate the degree of eccentricity based upon the detected changes.
Based upon signals from a robot arm controller 68, the wafer is periodically lifted by the robot arm 28 to reposition the wafer so that its center 54 is aligned with the spinner center 56. Once the wafer is ally centered, the edge detector 60 is then focused upon the notch 58. The wafer is spun until the notch is found. The spinner 48 rotates the wafer so that the notch is positioned at a desired rotation orientation. Following the centering and orienting procedure, the wafer is lifted from the spinner 48 using, for example, the pins 52 and reengaged by the arm end effector 34. The end effector then transports the wafer (arrow 68) to the process module 40. At this time, the end effector enters the module and places the wafer at a precise, preprogrammed location for semiconductor processing. Following processing, the wafer is typically moved by the arm (arrow 70) to a receive indexer 72. The wafer may be realigned in the alignment module each time a further process is performed. Thus, if the wafer is subsequently moved to the second process module 42, it may, first, make a return trip to the alignment module 46.
In some facilities, process modules include elaborate entrance and exit load locks (ELL's and XLL's) each having individual arms with individual alignment units. Clearly, a great deal of space is occupied by the need for dedicated alignment modules and mechanisms. As noted above, this waste of precious clean room floor space to provide alignment devices adds substantial costs to the process.
It is, therefore, an object of this invention to provide a method and apparatus for aligning and rotationally orienting substrates, such as semiconductor wafers, that omit complex alignment modules and mechanisms. This method and the related apparatus should increase the overall process speed for manufacturing wafers, but should still provide desired information on the precise orientation and alignment of individual wafers. The method and apparatus should also provide improved tracking of wafers as they move through a processing facility.